Glaciers are formed when snow builds up for several years from snowfall that exceeds snow melt. Over time, as new snow compresses the existing snow, the weight causes the snow underneath to turn to ice. The hardening effect is similar to what happens when you pack a snowball in your hand. The pressure from additional layers of snow builds for up to thousands of years; the ice gets compressed so much that the air is forced out and the glacier's color appears to be blue.
Glaciers leave striations, which are like long deep scratches in the bedrock of the Earth, and moraines, which are ridges in the soil made up of rock debris. Sheepbacks and drumlins are similar-shaped asymmetrical rock formations that face opposite directions. Horns and aretes are steep rock formations, formed when multiple glaciers come together, and cirques are formed when bedrock that has collapsed under a glacier forms a resulting basin.
The extreme weight of glaciers puts pressure on the land underneath. Glaciers press on the Earth's crust, carve out valley floors, make lakes deeper, expand rivers, gouge out rocks and pick them up as they move. They crush rocks into powder called rock flour and expand certain geological features when they pass over. The Antarctic ice cap is so heavy, it compresses the earth slightly at the South Pole, giving it a slight pear shape.
What's most important about glacier formation is that glaciers are dynamic forces of nature. They change with climate and conditions. That's also a problem, because warming global temperatures are causing many glaciers to retreat -- less snow accumulates at higher levels than melts at lower ones [source: Physical Geography].
At Glacier National Park in West Glacier, Montana, there were 150 glaciers in 1850, but only 26 today, and if the global warming trend continues at its current pace, there will be no glaciers left by 2020 [source: National Park Service]. The park is a site of studies on glacier retreat by the U.S. Geological Survey to project climate change effects on glaciers around the world. Scientists can study the glaciers' maximum sizes from their past histories because of the mounds of rock as soil left behind in the form of moraines. The melting glaciers in Glacier National Park could affect the area's wildlife, aquatic food chains and even tourism [source: USGS].
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